nor have heard of anyone else having been charged either
I've heard stories.
I think the policy has slightly changed now that they only charge for work done beyond the NTE if there's no fault found up to the NTE. A good ISP will put a note on the ticket explicitly stating that no work is to be done beyond the NTE.
Basically, you want to do all you can to ensure the job is
not classified as SFI ("special faults investigation") by BT.